The present invention relates to apparatus and methods for chemical mechanical polishing a substrate.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a patterned stop layer, and planarizing the filler layer until the stop layer is exposed. For example, trenches or holes in an insulative layer may be filled with a conductive layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer, which in turn sets the maximum throughput of the CMP apparatus.
During CMP operations, the polishing pad needs to be replaced periodically. For a fixed-abrasive pad, the substrate wears away the containment media to expose the embedded abrasive particles. Thus, the fixed-abrasive pad is gradually consumed by the polishing process. After a sufficient number of polishing runs (e.g., forty to fifty) the fixed-abrasive pad needs to be replaced. For a standard pad, the substrate thermally and mechanically damages the polishing pad and causes the pad""s surface to become smoother and less abrasive. Therefore, standard pads must be periodically xe2x80x9cconditionedxe2x80x9d to restore a roughened texture to their surface. After a sufficient number of conditioning operations (e.g., three hundred to four hundred), the conditioning process consumes the pad or the pad is unable to be properly conditioned. The pad must then be replaced. An advantage of fixed-abrasive polishing pads is that they do not need to be conditioned.
One problem encountered in the CMP process is difficulty in replacing the polishing pad. The polishing pad may be attached to the platen surface with an adhesive. Significant physical effort is often required to peel the polishing pad away from the platen surface. The adhesive then must be removed from the platen surface by scraping and washing with a solvent. A new polishing pad can then be adhesively attached to the clean surface of the platen. While this is happening, the platen is not available for the polishing of substrates, resulting in a decrease in polishing throughput. This problem is even more acute for fixed-abrasive pads, which need to be replaced more often than standard polishing pads. Thus, although the fixed-abrasive pads do not need to be conditioned, the use of fixed-abrasive pads in a CMP apparatus results in a higher cost of operation.
In one aspect, the invention is direct to a chemical mechanical polishing apparatus having a rotatable platen, a generally linear polishing sheet having an exposed portion extending over a top surface of the platen for polishing the substrate, and a drive mechanism to incrementally advance the polishing sheet in a linear direction across a top surface of the platen. The polishing sheet is releasably secured to the platen to rotate with the platen, and it has a width greater than a diameter of the substrate.
Implementations of the invention may include the following. An unused portion of the polishing sheet may be wrapped around a feed roller, and a used portion of the polishing sheet may be wrapped around a take-up roller. The drive mechanism may incrementally advance the polishing sheet by unwrapping a segment of the unused portion of the polishing sheet from the feed roller, and wrapping a segment of the exposed portion of the polishing sheet around the take-up roller. The feed roller and take-up rollers may be rotatably connected to the platen. A guard may prevent contamination of the unused portion of the polishing sheet. The guard may include a frame positioned near the feed roller, the frame suspended above the polishing sheet to form a gap, and a source of pressurized gas to direct gas through the gap.
First and second rollers may be rotatably connected to retainers on the platen, e.g., by pins extending through apertures in the retainers and into receiving recesses in the rollers. A spring may be positioned to urge the second roller to rotate in a first direction and pull the polishing sheet across the top surface of the platen. A ratchet may engage a gear that is connected to the first roller to prevent the first roller from rotating, thereby preventing the polishing sheet from moving across the top surface of the platen. An escapement clutch may be actuated to disengage the ratchet from the gear to permit the first roller to rotate. The ratchet and gear may be configured so that actuation of the escapement clutch permits the first roller to rotate by a predetermined incremental amount. A pneumatic line may be coupled to the escapement clutch, and the escapement clutch may be actuated by applying a pressure to the pneumatic line. A slip clutch may be coupled to the second roller to prevent the second roller from rotating in a second direction opposite to the first direction. The first and second rollers may be positioned below the top surface of the platen.
A platen drive motor may rotate the platen. A channel may be formed in the platen to vacuum-chuck the polishing sheet to the platen. The channel may extend along the edges of the platen. The top surface of the platen may have a generally rectangular shape. A transparent region may be formed in the polishing sheet, and the platen may include an aperture aligned with the transparent region. An optical monitoring system may direct light through the aperture and the transparent region to impinge the substrate. The polishing sheet may be a fixed-abrasive polishing material or a non-fixed-abrasive polishing material. A carrier head may hold the substrate, and a carrier head drive motor may rotate the carrier head.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus with a plurality of polishing stations and a carousel supporting a plurality of polishing heads above the plurality of polishing stations. Each polishing station including a rotatable platen base adapted to receive either a circular platen, or a rectangular platen and a drive mechanism to incrementally advance a polishing sheet in a linear direction.
Implementations of the invention may include the following. The plurality of polishing stations may include a first polishing station with a first platen, a fixed-abrasive polishing sheet and a linear drive mechanism to incrementally advance a fixed-abrasive polishing sheet across a top surface of the first platen, and a second polishing station with a second platen and a polishing sheet suitable for buffing adhesively attached to the second platen.
In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate is brought into contact with a generally linear polishing sheet that extends over a top surface of a rotatable platen. The polishing sheet having a width greater than a diameter of the substrate. The polishing sheet is releasably secured to the platen, and the platen is rotated to rotate the polishing sheet and create relative motion between the substrate and the polishing sheet. The polishing sheet is released from the platen and incrementally advanced in a linear direction across the top surface of the platen after the polishing step has been completed.
Implementations of the invention may include the following. The polishing sheet may be vacuum-chucked to the platen. An unused portion of the polishing sheet may be unwrapped from a feed roller and a used portion of the polishing sheet may be wrapped around a take-up roller. The substrate may be rotated with a carrier head.
Advantages of the invention may include the following. More substrates can be polished without replacing the polishing pad, thereby reducing downtime of the CMP apparatus and increasing throughput. An sheet of advanceable fixed-abrasive polishing material can be provided in a polishing cartridge. It is easy to remove and replace the polishing cartridge from a platen. Either a circular platen or a rectangular platen (to which the polishing cartridge would be attached) can be mounted at each polishing station of the CMP apparatus. The polishing apparatus gains the advantages associated with fixed-abrasive polishing materials. A rotating carrier head can be used to press the substrate against the rotating polishing sheet.
Other features and advantages will be apparent from the following description, including the drawings and claims.